The absence rules
To apply for British citizenship after Indefinite Leave to Remain (or settled status), GOV.UK says you must have lived in the UK for 5 years before applying, been outside the UK for no more than 450 days in those 5 years, and no more than 90 days in the last 12 months.
You normally must have held ILR or settled status for 12 months, unless you are married to a British citizen.
How days are counted
Only whole days abroad count: the day you leave and the day you return are not absences. If you are slightly over a limit, the Home Office can sometimes use discretion, but it is not guaranteed. Moving your application date later lets old trips drop out of the period.
Frequently asked questions
Does the travel day count as an absence?
No. Only whole days outside the UK are counted.
I’m over 450 days. What can I do?
Try a later application date: each month, older trips may leave the 5-year period.
Is this enough to apply?
No. There are other requirements, such as the Life in the UK test, English and good character.
Are my dates saved?
No. Everything runs in your browser.
Sources
A counting aid. The Home Office makes the decision. Disclaimer.